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Somewhat related to How many plants would be needed to produce oxygen enough for 20 humans?

Suppose a number of humans are closed in a large hermetically sealed room on Earth with a glass (or other see-through) roof material that allows sunlight through:

Would it be possible to create a symbiotic relationship between humans and plants / trees whereby the humans would exhale enough CO2 for the plants to survive whilst the plants would produce enough oxygen for the humans to survive?

Would (for example) one person and one beech / oak tree work (or some combination of trees that don't shed leafs seasonally and continue absorbing enough CO2 whilst emitting enough oxygen all year around)? If not, how many humans and how many trees / plants would work?

I am thinking if a disaster strikes Earth and the atmosphere becomes unbreathable whether such a system could theoretically work?

Also, this set-up would be highly relevant for Mars whilst also being very highly relevant for any type of self-sustainable orbiter or spaceship (and therefore relevant for space exploration).

Thank you for any responses or tips,

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    $\begingroup$ It can be done, although our attempts so far have stumbled. See Biosphere $\endgroup$ Jul 3, 2021 at 12:32
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    $\begingroup$ There is no way that deciduous trees like oak and beech could support human life in a sealed environment, in any number, unless the humans find a way to stop breathing for a few months while the trees are dormant with no leaves in winter. (And if you try to make deciduous trees grow continuously all year round, that will also fail.) $\endgroup$
    – alephzero
    Jul 4, 2021 at 13:01
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    $\begingroup$ @PM2Ring IMO, sustainable production of air, water, and food in a closed system is the most significant challenge in long-term space exploration, and it remains as perpetually out of reach as fusion-powered rockets. $\endgroup$ Jul 4, 2021 at 13:55
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    $\begingroup$ @PM2Ring Stealing words from Inigo Montoya (as portrayed by Mandy Patinkin), "Hello. My name is outer space. You killed the Earth. Prepare to die." $\endgroup$ Jul 4, 2021 at 14:23
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    $\begingroup$ @Cornelis: have edited the title and the body a bit to address your concern: I do think the question is highly relevant for space exploration, because (for example), plants are meant to play a significant role in oxygen production and CO2 extraction for any future Mars settlements (see this design here, for example). $\endgroup$ Jul 5, 2021 at 11:14

2 Answers 2

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This is being done with biosphere 2 and several similar projects and found to be very complicated. Specifically in a small sealed system there is little buffering or inertia available if one element is over or under producing or just having a seasonal change so die offs are hard to avoid.

For this reason the plants on the ISS and proposed future projects generally have some form of life support of their own to keep them optimally healthy, with that mechanical/chemical life support system buffering the changes in throughput.

Trees are sub optimal choice as life support in terms of CO2 processed per unit mass (unsourced 'seven or eight trees worth per person') with systems such as MELiSSA using algae and 'grasses' (rice/wheat) along with a heated reaction chamber and a bacterial nitrogen processing bed to reduce the footprint and improve control. Algae and Bacteria respond much better to being slowed down or speed up as humans and other elements change production than trees do.

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    $\begingroup$ Biosphere 2 didn't quite work as planned. Another way to say it: It was a failure. $\endgroup$ Jul 3, 2021 at 16:17
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    $\begingroup$ @DavidHammen. The first mission gathered enough data to make the second one possibly succeed, except we can't have nice things $\endgroup$ Jul 4, 2021 at 5:35
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    $\begingroup$ @LioElbammalf I'm with David Hammen, possibly because I am old enough to have watched the train wreck that was the financial back end of Bio 2 as it happened and deleted parts of my answer on it 'being successful at separating people from their money' as irrelevant to the question. Certainly the science got lost a bit in pursuit of 100% efficiency and PR (we already know a lot about human malnutrition). $\endgroup$ Jul 4, 2021 at 10:16
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    $\begingroup$ @LioElbammalf The first of the two Biosphere 2 expeditions ran into several severe problems. Oxygen levels eventually fell from 20% to 14%. That was equivalent to living atop a 13000 foot mountain. The people inside lost a lot of weight, partly because of the reduced oxygen levels, and partly because the limited food they grew placed them on a near starvation-level diet. Food and oxygen were eventually smuggled in. Yet another issue was small isolated group dynamics. People who serve in submarines or in the ISS are carefully screened for psychological issues. Biosphere 2 did not do that. $\endgroup$ Jul 4, 2021 at 13:04
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    $\begingroup$ The second expedition was even more shambolic, thanks in part to actions by the then acting CEO Steve Bannon. (Yes, that Steve Bannon.) Failure is a one word summary of Biosphere 2. A two word summary is "hot mess." $\endgroup$ Jul 4, 2021 at 13:07
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Terrestrial land plants are not sufficient producers of spare oxygen. Marine Bacteria, algae produce majority of Earth's freely available oxygen. Terrestrial plants produce barely fraction. So membrane bioreactors with algae cultivation produce sufficiently available oxygen.

early experimentation with BIOS-1 they demonstrated you can produce all the oxygen an adult male. Would require you woukd need for one person from approximately 20 kilograms of water and algae. However it would require sufficient sunlight thus would have to be spread over 8 square meters of surface area, about 90 square feet.

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